Alloy of iron, aluminum, and chromium



United States Patent OfiFice 3,068,094 Patented Dec. 11, 1%62 3 068,094 ALLOY OF IRON, ALTJMINUM, AND CHROMIUM Victor F. Zackay, Dearborn, and Richard E. Heimerdinger, Detroit, Mich., assignors to The Ford Motor Company, Dearborn, Mich., a corporation of Delaware No Drawing. Filed Jan. 27, 1959, Ser. No. 789,240 2 Claims. (Cl. 75-124) This invention is concerned with the field of siderurgy and is more particularly concerned with an alloy of iron, aluminum and chromium which exhibits remarkable resistance to carburization, sulphurization, corrosion and very high temperature oxidation and concomitantly retains a high degree of ductility and workability. The alloys described herein constitute an improvement on the alloys described and claimed in United States Letters Patent 2,804,387, granted August 27, 1957, to Morgan and Zackay.

Alloys of iron, chromium and aluminum containing 6.5 to 8.0-percent aluminum and 7.0 to 8.0 percent chromium have been found to possess remarkable resistance to carburization, sulphurization, corrosion and very high temperature oxidation combined wtih reasonable ductility provided they are prepared in a manner which assures a rigidly deoxidized and degasified product. Vacuum melting procedures may be employed to this end, but commercial considerations require that such an alloy be pre pared by an air melting technique. The exact procedure by which this rigid deoxidation is finally consummated is immaterial provided there is eventually produced a molten iron, aluminum chromium alloy containing a very low percentage of oxygen and this molten alloy is poured in a manner to retain this low oxygen content.

In the preparation of this alloy, electrolytic iron and ferro chromium are melted in a rammed magnesia crucible without the benefit of any slag covering. While not so limited, it is preferred that this melting be accomplished by use of high frequency currents. While the electrolytic iron and ferro chromium are being melted, a bath of molten aluminum is prepared in an ordinary teapot graphite crucible. This aluminum is held at a temperature of about 1800 F.

After melting of the electrolytic iron and ferro chromium is complete, a preliminary deoxidation is effected by the addition of 0.1% aluminum and 0.5% manganese, both being added as pure metals or as ferroalloys. A more drastic deoxidation of the molten iron chromium bath is now required. This is accomplished by exposing the melt to metallic calcium. This may best be done by wiring small cubes of metallic calcium to an iron rod and thrusting the iron rod into the melt. Calcium is preferred but for this purpose it may be replaced by an aluminum calcium alloy or by magnesium or any suitable combination of these elements. The violent ebullition of gas from the deoxidizing metal serves to purge and violently agitate the bath. By way of example, a total of 0.5 of calcium is added to the molten iron chromium bath in two stages. The second of the two stages is carried out just before the mixing of the iron chromium bath and aluminum so that a slight excess of calcium remains in the melt.

In a manner similar to that described for the iron chromium bath the aluminum is now deoxidized using approximately 0.1% calcium, based upon the amount of the aluminum. This deoxidized aluminum is then further degassed by introducing chlorine into the melt through a refractory tube. This chlorination serves to eliminate any hydrogen which may have been generated by the reaction of the aluminum and the water vapor of the air. The reaction of the chlorine with the aluminum bath is exothermic and tends to raise the temperature of the bath. After the aluminum is completely degassed the oxidized dross is skimmed off, fluorspar is added to the iron chromium bath after which the aluminum is slowly poured through the fluorspar slag into the iron chromium bath. All fluorspar used in this process should be rigidly anhydrous to avoid reaction between the aluminum and Water vapor.

Some oxidation of the aluminum during the pouring operation is inevitable. However, this aluminum oxide reacts with the fiuorspar slag again exothermically and tends to heat the bath. Precautions should be taken to avoid any toxic effects from the aluminum fluoride which is generated at this point. It is important during and after the addition of the molten aluminum that the bath be vigorously agitated. Induction stirring per se has not been found adequate for this purpose and resort has been had to vigorous mechanical stirring.

One minute prior to pouring the agitation is stopped and the power shut off. This permits the molten slag to stratify so that the bulk of the slag can be removed. Cast iron molds which have been given a wash of a chlorinated pitch or exposed to a carbon tetrachloride atmosphere are used to receive this metal. It is essential that the alloy be poured into the mold promptly to prevent harmful oxidation in the crucible. The metal is poured into these Washed cast iron molds at about 2900 F. The hot topping procedure must take into account the very deep piping encountered in this type of alloy.

Promptly upon solidification the ingots are stripped from the molds and buried in a heat-insulating medium such as vermiculite or lime. This is necessary to prevent thermal cracking because of the poor thermal conductivity and high thermal coefiicient of expansion of iron aluminum alloys.

An alloy typifying this invention has been prepared containing 7.5 percent aluminum, 7.5 percent chromium, 0.026 percent carbon, remainder iron and incidental impurities. In the oil quenched condition this alloy exhibited a tensile strength of 77,700 pounds per square inch, a yield strength of 60,400 pounds per square inch, an elongation of 35 percent and a reduction of area of 74 percent. The corresponding figures for the furnace cooled condition are 79,800 pounds per square inch, 59,- 600 pounds per square inch, 26 percent and 58 percent.

In commercial practice it is diflicult to exclude silicon from the melts. Furthermore, this element tends to enhance the resistance of the alloy to oxidation and carburization. This additional resistance to oxidation and carburization is obtained only at a heavy sacrifice of ductility as expressed in the elongation value. An alloy containing 7.71 percent aluminum, 7.54 percent chromium, 0.05 percent carbon, 0.34 percent silicon, remainder substantially all iron exhibited an elongation of 17 percent in the oil quenched condition and 12 percent in the furnace cooled condition.

The strength of the ferrite matrix in this alloy may be further strengthened by the addition of small amounts of metals which are strong carbide formers. Examples of such metals are molybdenum, titanium, niobium, and vanadium. These carbide formers should be added with caution because of their catastrophic effect upon ductility, particularly in the furnace cooled condition. An alloy comprising 7.90 percent aluminum, 6.75 percent chromium, 0.57 percent titanium, 0.02 percent carbon and 0.35 percent silicon exhibited an elongation in the oil quenched condition of 27 percent and in the furnace cooled condition of only 2 percent. A similar alloy containing 7.5 percent aluminum, 7.90 percent chromium, 1.07 percent titanium, 0.03 percent carbon and 0.35 percent silicon exhibited an elongation in the oil quenched condition of 20 percent and in the furnace cooled condition only 1 percent.

Accordingly, when the term iron is employed in the at- 3) tached claims it is to be understood to comprehend also iron alloyed with minor amounts of silicon, the carbide forming metals and impurities of an incidental nature.

The critical nature of the aluminum additions is shown from the fact that an alloy comprising 8.28 percent aluminurn, 6.93 percent chromium, 0.03 percent carbon and balance substantially all iron exhibited in the oil quenched condition an elongation and reduction of area of 15 percent and in the furnace cooled condition of 21 and 27 percent, respectively. To resist even one hundred hours exposure to air at 2000 F. a minimum aluminum content of 6 percent is necessary. No numerical data is available to support this statement, but it may be clearly demonstrated in colored photographs which are not included because of the difiiculties in reproducing colored photographs. Similarly it can be demonstrated by means of colored photographs that alloys containing 8.0 percent aluminum and up to 4.75 percent chromium, balance substantially all iron are heavily damaged by exposure to air at 2200 F. for 500 hours. The same test does no substantial harm to a specimen containing 7.25 percent chromium, and heavily damages one containing 9.50 percent chromium.

We claim as our invention:

1. A sulphur and carbon resistant ductile and Workable alloy of iron, chromium and aluminum consisting essentially of 6.5 to 8.0 percent aluminum, 7.0 to 8.0 percent chromium, a small but significant residue of a metal having an afiinity for oxygen at the melting point of the alloy much stronger than that of aluminum and chromium, the remainder essentially iron, said alloy being free from contamination by oxides of aluminum and chromium and having an oxygen content no higher than that which corresponds to the equilibrium between calcium and oxygen at the melting point of the alloy, the oxidation resistance of this alloy being such that it is substantially unaffected by exposure to air at 2200 F. for a period of 500 hours.

2. A sulphur and carbon resistant ductile and workable alloy of iron, chromium and aluminum consisting essentially of 7.5 percent aluminum, 7.5 percent chromium, a small but significant residue of a metal having an affinity for oxygen at the melting point of the alloy much stronger than that of aluminum and chromium, the remainder essentially iron, said alloy being free from contamination by oxides of aluminum and chromium and having an oxygen content no higher than that which corresponds to the equilibrium between calcium and oxygen at the melting point of the alloy, the oxidation resistance of this alloy being such that it is substantially unafiected by exposure to air at 2200" F. for a period of 500 hours.

References Cited in the file of this patent UNITED STATES PATENTS 2,703,355 Hagglund Mar. 1, 1955 2,804,387 Morgan et a1 Aug. 27, 1957 2,941,883 Ida et al June 21, 1960 2,987,394 Mueller et al. June 6, 1961 OTHER REFERENCES Basic Open Hearth Steelmaking," 1944, page 499. Published by the American Institute of Mining and Metallurgical Engineers. 

1. A SULPHUR AND CARBON RESISTANT DUCTILE AND WORKABLE ALLOY OF IRON, CHROMIUM AND ALUMINUM CONSISTING ESSENTIALLY OF 6.5 TO 8.0 PERCENT ALUMINUM, 7.0 TO 8.0 PERCENT CHROMIUM, A SMALL BUT SIGNIFICANT RESIDUE OF A METAL HAVING AN AFFINITY FOR OXYGEN AT THE MELTING POINT OF THE ALLOY MUCH STRONGER THAN THAT OF ALUMINUM AND CHROMIUM, THE REMAINDER ESSENTIALLY IRON, SAID ALLOY BEING FREE FROM CONTAMINATION BY OXIDES OF ALUMINUM AND CHROMIUM AND HAVING AN OXYGEN CONTENT NO HIGHER THAN THAT WHICH CORREPONDS TO THE EQUILIBRIUM BETWEEN CALCIUM AND OXYGEN AT THE MELTING POINT OF THE ALLOY, THE OXIDATION RESISTANCE OF THIS ALLOY BEING SUCH THAT IT IS SUBSTANTIALLY UNAFFECTED BY EXPOSURE TO AIR AT 2200*F. FOR A PERIOD OF 500 HOURS. 